BUG/MAJOR: ring: use the correct size to reallocate startup_logs

BUG/MAJOR: ring: use the correct size to reallocate startup_logs

In 3.0-dev, with commit 7c9ce715c9 ("MINOR: ring: make callers use
ring_data() and ring_size(), not ring->buf"), we made startup_logs_dup()
use ring_size() to get the old ring size and pass it to ring_new() to
create a new ring. But due to the ambiguity of the allocate vs usable
size, this resulted in slightly shrinking the buffer compared to the
previous one, occasionally causing crashes if the first one was already
full of warnings, as seen in GH issue #2529. We need to use the allocated
size instead, thanks to the function brought by previous commit.

No backport is needed, this only affects 3.0-dev. Thanks to @felipewd
for the detailed report that allowed to spot the problem.